In recent years, a variable valve actuation control (VVC) system, capable of variably adjusting a valve lift and valve timing of at least one of intake and exhaust valves of an internal combustion engine depending on engine operating conditions, is widely utilized for controlling a charging efficiency, an effective compression ratio, and an amount of residual gas of the engine, thereby enhancing the combustion performance and engine power performance and exhaust emission control performance. In gasoline engines, the premixed air-fuel mixture is ignited by means of a spark plug. In Diesel engines or premix compression ignition engines, air is compressed during the compression stroke, cylinder, is self-ignited due to a temperature rise of the and then fuel, which is sprayed or injected into the compressed gas (heat produced by compressing the incoming air).
The ignition lag, which controls or manages the state of combustion, changes depending on the temperature and pressure of air-fuel mixture, and various states or various characteristics of air-fuel mixture, for example, a turbulence intensity, and a fuel property. In other words, a deviation of the ignition lag from an optimal value varies depending on the air-fuel mixture temperature, air-fuel mixture pressure, turbulence intensity in the combustion chamber, fuel property, and the like. By means of the VVC system, it is possible to adjust the effective compression ratio, thereby compensating for a deviation of the ignition lag from an optimal value. As a consequence, it is possible to optimally control the state of combustion by means of the VVC system. Note that the effective compression ratio is correlated to a geometrical compression ratio but differs from the geometrical compression ratio. The geometrical compression ratio often denoted by Greek letter “ε” is generally defined as a ratio (V1+V2)/V1 of the full volume (V1+V2) existing within the engine cylinder and combustion chamber with the piston at a bottom dead center (BDC) position to the clearance-space volume (V1) with the piston at a top dead center (TDC) position. On the other hand, the effective compression ratio denoted by Greek letter “ε′” is generally defined as a ratio of the effective cylinder volume corresponding to the maximum working medium volume to the effective clearance volume corresponding to the minimum working medium volume. These two compression ratios ε and ε′ are thermodynamically distinguished from each other. The state of combustion is affected by various factors as well as effective compression ratio ε′, for example, the air-fuel mixture temperature, and thermodynamic and hydrodynamic properties or characteristics (boost pressure created by a super-charging system, super-charged air temperature, cooling characteristics in a cooling system, an amount of deposits adhered to the cylinder wall, a fuel injection pattern of one cycle, a swirl intensity of air-fuel mixture, an amount of residual gas, an amount of external EGR (exhaust gas recirculated), and the like). Additionally, for the same intake valve closure timing, the effective compression ratio ε′ is affected by several factors, for example the air-fuel mixture temperature at the beginning of compression stroke, the air-fuel mixture pressure at the beginning of compression stroke, and the EGR amount.
There have been proposed effective compression ratio based engine control technologies, for example, an effective compression ratio ε′ increase control during an engine starting period for the enhanced ignitability, and an effective compression ratio ε′ decrease control after starting for the reduced mechanical friction loss of the engine.
It has already been proposed that, in Diesel engines, effective compression ratio ε′ is controlled by variably adjusting an intake valve closure timing depending on engine operating conditions, to optimize the state of combustion. One such combustion control technology with variable valve timing (i.e., with effective compression ratio ε′ control) has been disclosed in Japanese document “2005 JSAE Annual Congress, No. 20055167, Yokohama, Japan, May 18, 2005” published by Society of Automotive Engineers of Japan, Inc. and titled “Achievement of Medium Speed and Load Premixed Diesel Combustion with Variable Valve Timing” and written by authors Yutaka Murata et al.
Additionally, there have been proposed and developed methods needed to optimize or fit the control variable in a premixed compression ignition engine model using measured instantaneous gasoline-engine cylinder pressures. One such model-based gasoline engine control technology has been disclosed in United States document “SAE Paper 2005-01-0752, SAE International, Apr. 11-14, 2005” published by Society of Automotive Engineers, Inc. and titled “Gasoline Engine Operation with Twin Mechanical Variable Lift (TMVL) Valvetrain, Stage 1: SI and CAI Combustion with Port Fuel Injection” and written by authors J. Stokes et al.
Furthermore, there have been proposed engine control methods for optimizing or fitting fuel injecting conditions based on a measured value of the state of combustion, such as in-cylinder pressure, in order to compensate for individual differences of variable valve actuation control (VVC) devices manufactured and/or deteriorations of VVC devices with age. One such in-cylinder pressure dependent fuel-injection optimization method has been disclosed in Japanese document “2005 JSAE Annual Congress, No. 20055184, Yokohama, Japan, May 18, 2005” published by Society of Automotive Engineers of Japan, Inc. and titled “Development of a Gasoline HCCI Engine Control System” and written by authors Hiromu Kakuya et al.
As is generally known, it is possible to variably adjust the mass of air entering the engine cylinder at the beginning of the compression stroke by retarding or advancing the intake-valve closure timing, denoted by “IVC” and expressed in terms of crank angle. In such a case, it is possible to retard a rise in in-cylinder pressure and a rise in in-cylinder temperature with respect to a predetermined crank angle. In other words, it is possible to lower effective compression ratio ε′ by retarding an in-cylinder pressure rise and/or an in-cylinder temperature rise by way of variable adjustment of intake valve closure timing IVC. One such IVC adjustment type variable compression ratio device for a compression ignition engine has been disclosed in Japanese Patent Provisional Publication No. 1-315631 (hereinafter is referred to as “JP1-315631”). In the case of JP1-315631, the IVC adjustment type variable compression ratio device is exemplified in a two-stroke-cycle Diesel engine. Concretely, when it is determined that the current operating condition of the two-stroke-cycle Diesel engine corresponds to an engine starting period, intake valve closure timing IVC is phase-advanced towards a timing value near bottom dead center (BDC) by means of an electric-motor driven variable valve operating device (or a motor-driven variable valve timing control (VTC) system), thereby increasing an effective compression ratio and consequently enhancing the self-ignitability during the starting period. In contrast, during engine normal operation, intake valve closure timing IVC is phase-retarded to decrease the effective compression ratio and consequently to reduce a fuel consumption rate. The motor-driven VTC system of JP1-315631 uses a rotary-to-linear motion converter, such as a ball-bearing screw mechanism, for changing relative phase of an intake-valve camshaft to an engine crankshaft. The rotary-to-linear motion converter (the ball-bearing screw mechanism) of JP1-315631 is comprised of a warm shaft (i.e., a ball bearing shaft with helical grooves) driven by a step motor, an inner slider (i.e., a recirculating ball nut), recirculating balls provided in the helical grooves, and an outer slider axially movable together with the inner slider and rotatable relative to the inner slider. The other types of variable valve operating devices have been disclosed in (i) Japanese document “JSAE Journal Vol.59, No. 2, 2005” published by Society of Automotive Engineers of Japan, Inc. and titled “Gasoline Engine: Recent Trends in Variable Valve Actuation Technologies to Reduce the Emission and Improve the Fuel Economy” and written by two authors Yuuzou Akasaka and Hajime Miura, and (ii) Japanese document “Proceedings JSAE 9833467, May, 1998” published by Society of Automotive Engineers of Japan, Inc. and titled “Reduction of the engine starting vibration for the Parallel Hybrid System” and written by four authors Hiroshi Kanai, Katsuhiko Hirose, Tatehito Ueda, and Katsuhiko Yamaguchi. The Japanese document “JSAE Journal Vol.59, No. 2, 2005” discloses various types of variable valve operating systems (various VVC systems), such as a helical gear piston type two-stepped phase control system, a rotary vane type continuously variable valve timing control (VTC) system, a swing-arm type stepped valve lift and working angle variator, a continuously variable valve event and lift (VEL) control system, and the like. The VTC and VEL control systems are operated by means of respective actuators for example electric motors or electromagnets, each of which is directly driven in response to a control signal (a drive signal) from an electronic control unit (ECU). Alternatively, the VTC and VEL control systems are often operated indirectly by means of a hydraulically-operated device, which is controllable electronically or electromagnetically. On the other hand, the Japanese document “Proceedings JSAE 9833467, May, 1998” teaches the use of a variable valve timing control system installed on the intake valve side of an engine of a hybrid vehicle employing a parallel hybrid system, for prevention of rapid engine torque fluctuations, which may occur during engine stop and start operation.